It is well known that rubber-reinforced polymers of monovinylidene aromatic compounds, such as styrene, alphamethylstyrene and ring-substituted sytrenes are desirable for a variety of uses. More particularly, rubber-reinforced polymers of styrene having included therein discrete particles of a cross-linked rubber, for example, polybutadiene, the discrete particles of rubber being dispersed throughout the styrene polymer matrix, can be used in a variety of applications including refrigerator linings, packaging applications, furniture, household appliances and toys. The conventional term for such rubber-reinforced polymers is "high impact polystyrene" or "HIPS". The physical characteristics and mechanical properties of HIPS are dependent upon many factors including the particle size of the cross-linked rubber particles. More particularly, the size of cross-linked rubber particles can affect such properties as environmental stress crack resistance (hereinafter referred to as ESCR), tensile strength, flexural strength and impact strength. For example, when the cross-linked rubber particles are small in size, properties such as tensile strength and gloss of the HIPS product are maximized, but percent elongation at break and impact strength are diminished. Conversely, when the rubber particle size is increased, properties such as tensile strength and gloss are reduced.
Numerous experimental efforts have been made to determine what size or size distribution of cross-linked rubber particles dispersed throughout a polystyrene matrix would be the most desirable for particular applications. However, even though much experimentation has occurred and desirable HIPS products have been produced, known production processes have a major drawback which relates generally to inaccurate control of rubber particle size in the HIPS product. Thus, even though certain grain sizes of the rubber particles have been found to produce desirable results, reproducing similar products on a daily production basis has heretofore been very difficult. Moreover, when it was desired to change a HIPS product slightly, corresponding changes in the production process have been difficult.
One proven process for producing HIPS products has been the batch or suspension process. The batch or suspension process involves the use of a single reaction vessel where polymerization occurs. Batch process formation of HIPS products has desirable aspects which include ease of control of rubber particle size due to the ability to monitor the single reaction vessel throughout the polymerization process and accordingly, high reproducible results are inherent from the batch process because of such monitoring capabilities. However, inherent shortcomings of the batch process are the amount of yield from the process and the downtime of the single reaction vessel. Thus, the batch process is not commercially feasible.
The commercial difficulties associated with the batch process have resulted in many experimenters attempting to utilize some type of continuous flow process for the manufacture of HIPS. Each of the known continuous flow processes utilize a plurality of reaction vessels wherein polymerization increases from one vessel to the next. For example, Bronstert et al (U.S. Pat. No. 3,658,946), Ruffing et al (U.S. Pat. No. 3,243,481), Wang et al (U.S. Pat. No. 4,451,612), Dow, (European Patent Application 0,096,447), and (Dow PCT Application U.S. Pat. No. 80/00325) are all examples of various attempts to produce HIPS by different continuous flow processes. Each of these known continuous flow processes have somewhat different objectives. However, each of the known processes suffers from the above-discussed drawbacks, namely, difficulties associated with reproducibility of results, the problems attendant physical changes in the HIPS products and rubber grain size control problems. For example, because many more reaction vessels are utilized in a continuous flow process, as compared to the batch process which utilizes a single reaction vessel, there are many more parameters to control and thus numerous opportunities for process modifications which can result in different HIPS products on a daily or hourly basis. Thus, there has been a long-felt need in the art to determine what is responsible for such changes in products and a corresponding need to provide a process which is controllable and can reproduce desired HIPS products reliably. It is important to control the size of the cross-linked discrete rubber particles contained in the polystyrene matrix because properties such as ESCR, IZOD impact strength, percent elongation and tensile strength, are directly affected by the particle size of the rubber particles.